React Native Text Size

Prior to layout, correctly measure the text and obtain font information from your app (Android and iOS).

There are two primary operations: “flatHeights,” which may be used to concurrently determine the height of several blocks of text and is best for <FlatList> or <RecyclerListView>.

The second one, “measure,” provides specific information on a single block of text:

• The width used by the text, with an option to calculate the real width of the largest line.
• The height of the text, with or without paddings.
• The number of lines.
• The width of the last line.
• Extended information of a given line.

The width and height are practically the same as those received from the onLayout event of a <Text> component with the same properties.

In both functions, the text to be measured is required, but the rest of the parameters are optional and work in the same way as with React Native:

• fontFamily
• fontSize
• fontWeight
• fontStyle
• fontVariant (iOS)
• includeFontPadding (Android)
• textBreakStrategy (Android)
• letterSpacing
• allowFontScaling
• width: Constraint for automatic line-break based on text-break strategy.

In addition, the library includes functions to obtain information about the fonts visible to the App.

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Installation

Mostly automatic installation from npm

yarn add react-native-text-size
react-native link react-native-text-size

Change the compile directive to implementation in the dependencies block of the android/app/build.gradle file.

Requirements:

• React Native v0.57 or later.
• Android API 16 or iOS 9.0 and above.

For versions prior to 0.56 of React Native, please use react-native-text-size v2.1.1

See Manual Installation on the Wiki as an alternative if you have problems with automatic installation.

API

• measure

• flatHeights

• specsForTextStyles

• fontFromSpecs

• fontFamilyNames

• fontNamesForFamilyName

measure

measure(options: TSMeasureParams): Promise<TSMeasureResult>

This function measures the text as RN does and its result is consistent* with that of Text’s onLayout event. It takes a subset of the properties used by <Text> to describe the font and other options to use.

If you provide width, the measurement will apply automatic wrapping in addition to the explicit line breaks.

* There may be some inconsistencies in iOS, see this Know Issue to know more.

Note:

Although this function is accurate and provides complete information, it can be heavy if the text is a lot, like the one that can be displayed in a FlatList. For these cases, it is better to use flatHeights, which is optimized for batch processing.

TSMeasureParams

Plain JS object with this properties (only text is required):

The sample App shows interactively the effect of these parameters on the screen.

TSMeasureResult

measure returns a Promise that resolves to a JS object with this properties:

lineInfo

If the value of the lineInfoForLine is greater or equal than lineCount, this info is for the last line (i.e. lineCount - 1).

In case of error, the promise is rejected with an extended Error object with one of the following error codes, as a literal string:

Example 1

//...
import rnTextSize, { TSFontSpecs } from 'react-native-text-size'

type Props = {}
type State = { width: number, height: number }

// On iOS 9+ will show 'San Francisco' and 'Roboto' on Android
const fontSpecs: TSFontSpecs = {
  fontFamily = undefined,
  fontSize = 24,
  fontStyle = 'italic',
  fontWeight = 'bold',
}
const text = 'I ❤️ rnTextSize'

class Test extends Component<Props, State> {
  state = {
    width: 0,
    height: 0,
  }

  async componentDidMount() {
    const width = Dimensions.get('window').width * 0.8
    const size = await rnTextSize.measure({
      text,             // text to measure, can include symbols
      width,            // max-width of the "virtual" container
      ...fontSpecs,     // RN font specification
    })
    this.setState({
      width: size.width,
      height: size.height
    })
  }

  // The result is reversible
  render() {
    const { width, height } = this.state
    return (
      <View style={{ padding: 12 }}>
        <Text style={{ width, height, ...fontSpecs }}>
          {text}
        </Text>
      </View>
    )
  }
}

flatHeights

flatHeights(options: TSHeightsParams): Promise<number[]>

Calculate the height of each of the strings in an array.

This is an alternative to measure designed for cases in which you have to calculate the height of numerous text blocks with common characteristics (width, font, etc), a typical use case with <FlatList> or <RecyclerListView> components.

The measurement uses the same algorithm as measure but it returns only the height of each block and, by avoiding multiple steps through the bridge, it is faster… much faster on Android!

I did tests on 5,000 random text blocks and these were the results (ms):

In the future I will prepare an example of its use with FlatList and multiple styles on the same card.

TSHeightsParams

This is an object similar to the one you pass to measure, but the text option is an array of strings and the usePreciseWidth and lineInfoForLine options are ignored.

The result is a Promise that resolves to an array with the height of each block (in SP), in the same order in which the blocks were received.

Unlike measure, null elements returns 0 without generating error, and empty strings returns the same height that RN assigns to empty <Text> components (the difference of the result between null and empty is intentional).

Example 2

//...
import rnTextSize, { TSFontSpecs } from 'react-native-text-size'

type Props = { texts: string[] }
type State = { heights: number[] }

// On iOS 9+ will show 'San Francisco' and 'Roboto' on Android
const fontSpecs: TSFontSpecs = {
  fontFamily = undefined,
  fontSize = 24,
  fontStyle = 'italic',
  fontWeight = 'bold',
}
const texts = ['I ❤️ rnTextSize', 'I ❤️ rnTextSize using flatHeights', 'Thx for flatHeights']

class Test extends Component<Props, State> {
  state = {
    heights: [],
  }

  async componentDidMount() {
    const { texts } = this.props
    const width = Dimensions.get('window').width * 0.8
    const heights = await rnTextSize.flatHeights({
      text: texts,      // array of texts to measure, can include symbols
      width,            // max-width of the "virtual" container
      ...fontSpecs,     // RN font specification
    })
    this.setState({
      heights
    })
  }

  render() {
    const { texts } = this.props
    const { heights } = this.state

    return (
      <View style={{ padding: 12 }}>
        {texts.map(
          (text, index) => (
            <Text style={{ height: heights[index], ...fontSpecs }}>
              {text}
            </Text>
          )
        )}
      </View>
    )
  }
}

specsForTextStyles

specsForTextStyles(): Promise<{ [key: string]: TSFontForStyle }>

Get system font information for the running OS.

This is a wrapper for the iOS UIFont.preferredFontForTextStyle method and the current Android Material Design Type Scale styles.

The result is a Promise that resolves to a JS object whose keys depend on the OS, but its values are in turn objects fully compatible with those used in the RN styles, so it can be used to stylize <Text> or <TextInput> components:

TSFontForStyle

To know the key names, please see Keys from specsForTextStyles in the Wiki.

I have not tried to normalize the keys of the result because, with the exception of two or three, they have a different interpretation in each OS, but you can use them to create custom styles according to your needs.

fontFromSpecs

fontFromSpecs(specs: TSFontSpecs): Promise<TSFontInfo>

Returns the characteristics of the font obtained from the given specifications.

TSFontSpecs

This parameter is a subset of TSMeasureParams, so the details are omitted here.

fontFromSpecs uses an implicit allowsFontScaling:true and, since this is not a measuring function, includeFontPadding has no meaning.

TSFontInfo

The result is a Promise that resolves to a JS object with info for the given font and size, units in SP in Android or points in iOS, using floating point numbers where applicable*.

* Using floats is more accurate than integers and allows you to use your preferred rounding method, but consider no more than 5 digits of precision in this values. Also, remember RN doesn’t work with subpixels in Android and will truncate this values.

See more in:

Understanding typography at the Google Material Design site.

About Text Handling in iOS for iOS.

fontFamilyNames

fontFamilyNames(): Promise<string[]>

Returns a Promise for an array of font family names available on the system.

On iOS, this uses the UIFont.familyNames method of the UIKit.

On Android, the result is hard-coded for the system fonts and complemented dynamically with the fonts installed by your app, if any.

See About Android Fonts and Custom Fonts in the Wiki to know more about this list.

fontNamesForFamilyName

fontNamesForFamilyName(fontFamily: string): Promise<string[]>

Wrapper for the UIFont.fontNamesForFamilyName method of UIKit, returns an array of font names available in a particular font family.

You can use the rnTextSize’s fontFamilyNames function to get an array of the available font family names on the system.

This is an iOS only function, on Android it always resolves to null.

Known Issues

Inconsistent width between platforms

In iOS, the resulting width of both, measure and flatHeights, includes leading whitespace while in Android these are discarded.

Incorrent height (iOS)

On iOS, RN takes into account the absolute position on the screen to calculate the dimensions. rnTextSize can’t do that and both, width and height, can have a difference of up to 1 pixel (not point).

letterSpacing not scaling (iOS)

RN does not support the Dynamic Type Sizes, but does an excellent job imitating this feature through allowFontScaling …except for letterSpacing that is not scaled.

I hope that a future version of RN solves this issue.

lineHeight Support

Although rnTextSize provides the resulting lineHeight in some functions, it does not support it as a parameter because RN uses a non-standard algorithm to set it. I recommend you do not use lineHeight unless it is strictly necessary, but if you use it, try to make it 30% or more than the font size, or use rnTextSize fontFromSpecs method if you want more precision.

Nested Text

Nested <Text> components (or with images inside) can be rasterized with dimensions different from those calculated, rnTextSize does not accept multiple sizes.

TODO

• Normalized tracking or letter spacing in font info.
• More testing, including Android and iOS TVs.
• Learn the beautiful English, to make better docs.
• Find something nice in the ugly Objective-C.

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Thanks for your support!

License

The BSD 2-Clause “Simplified” License.

© 2018-2019, Alberto Martínez. All rights reserved.